What causes tornadoes?
Tornadoes need a strong thunderstorm plus winds that change speed and direction with height. That shear rolls the air into a horizontal spin, the storm's updraft tilts it upright, and stretching it tightens the spin, like a skater pulling in their arms.
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The ingredients
- Warm, humid air near the ground, usually from the Gulf of Mexico.
- Cooler, drier air above, which makes the atmosphere unstable so air shoots upward.
- Wind shear: winds that turn and strengthen with height.
- A trigger such as a front or dry line to start the storms.
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From spinning storm to tornado
The most dangerous tornadoes come from supercells, thunderstorms with a rotating updraft called a mesocyclone, often a few miles wide. Only some supercells produce tornadoes; scientists are still working out exactly why one does and another doesn't, which is why warnings come with short lead times.
Where and when
The US gets more tornadoes than any other country, roughly 1,200 a year, because warm Gulf moisture, dry air from the Southwest and cold air from Canada meet over the flat Plains. Spring is the peak, shifting from the South in early spring to the northern Plains by summer. Tornadoes are rated on the Enhanced Fujita scale, EF0 to EF5, from the damage they leave.
Questions people ask
- What's the difference between a tornado watch and warning?
- A watch means conditions are right for tornadoes; a warning means one is happening or indicated on radar, so take shelter now.
- Where is the safest place in a tornado?
- The lowest floor of a sturdy building, in an interior room without windows, or a storm shelter. Cars and mobile homes are not safe.